Stray Current From DC Powered Transit Systems - Tom Mollica

Stray Current From DC Powered Transit Systems - Tom Mollica

1. Stray Current Interference corrosion from dc transit systems NACE TWIN CITIES SECTION PIPELINE CORROSION CONTROL SEMINAR February 5-6, 2019 Graduate Hotel , Minneapolis, MN Presented by: tom Mollica, iii piping and corrosion specialties, inc.

2. Metro transit light rail – Minneapolis-saint PAUL, MN.

3. Basic electrical circuit of a dc powered light rail system +/- 1 mile TPSS- Traction Power Sub Station TPSS- Traction Power Sub Station

4. Safeguards for electrical isolation Rail boots on embedded track Direct fixation track where possible Concrete ties for ballasted track instead of wood Dielectric components for rail fasteners Ballast dressing and minimum clearances Minimum track-to-earth resistances upon commissioning (250-1,000 ohms/track-ft.) Longitudinal bonding of reinforcement to act as a “safety net”

5. New Rail construction

6. Testing and commisioning

7. Rail electrical isolation components EMBEDDED TRACK RAIL BOOT TIE & BALLAST RAIL INSULATOR ASSEMBLY

8. Typical rebar bonding details

9. reality

10. Continued poor maintenance

11. continued

12. Definition of Stray Current Interference: The NACE International recommended practice on cathodic protection underground structures provides several insights to the definition and evaluation of interference. Stray current is defined as “current through paths other than the intended circuit” or “the deterioration of a material, usually a metal that results from a reaction with its environment”.

13. DC Transit Stray Current corrosion In 1995 electrified rail stray current was estimated to cost $500 million in damage annually One of the most common causes of stray current. System operates on 750 volts DC Can require up to 12,000 amperes of current

14. Corrosion Hot Spots Dynamic Stray Current Interference Areas of corrosion loss

15. Damage to rail components from stray current discharge

16. Corrosion Rates of Various Metals: Steel: 20 lbs./Amp-Year Copper 45 lbs./Amp-Year Aluminum 6.4 lbs./Amp-Year Lead 74 lbs./Amp-Year

17. How to protect from dynamic stray current damage Properly design electrical isolation for the track Maintain a high level of track-to-earth resistance Bond rebar in embedded areas to collect any stray current and evenly distribute across the steel surface Install adjacent utilities with a dielectric coating and electrical isolation to ensure a high resistance to earth of the structure making it difficult for the stray current to “see the structure Install cathodic protection on adjacent or crossing metallic structures Have a representative at the Corrosion Coordinating Meetings (be a good neighbor)

18. Keys to effective testing of adjacent utilities Need to test a regular intervals Utilize a data recorder to test in 24 hour increments to compare revenue vs. non-revenue data Realize the effects of this dynamic stray current are short lived but the exposure to large amounts of current are likely Make sure to maintain effective cathodic protection

19. Thank You for Your Attention Questions? Tom Mollica,III [phone] [email protected]